This invention relates to a comparison circuit, obtained according to MOS technology, for comparing two input voltages and comprising a differential amplifier,
to whose first input are supplied through a first capacitor during a first time period a first input voltage and during a second time period succeeding the first time period a second input voltage by means of a switch arrangement, PA1 whose second input is applied to a reference voltage and PA1 whose output is connected to the first input during the first time period.
Such a comparison circuit is used, for example, in analog-to-digital converters which operate according to the successive approximation principle. Such an analog-to-digital converter further consists of a digital-to-analog converter and of a control circuit which delivers the digital output signal. The control circuit sets at the start of a conversion process the bit of highest value, which is subjected to a digital-to-analog conversion and is compared in the comparison circuit with the analog input signal. If the comparison signal subjected to a digital-to-analog conversion is smaller than the analog input signal, the bit of highest value remains set, whereas it is reset if this signal is larger. The comparison is repeated successively with the next bit. Thus, the digital output signal is caused to approach in a stepwise manner the analog input signal.
The comparison circuit mentioned in the opening paragraph is known from the magazine "Markt und Technik", No. 9 of Mar. 1.sup.st 1985, p. 76. During the first time period, the difference between the first input voltage and the volume produced at the first input of the differential amplifier when the output and the first input are interconnected is applied to the first capacitor. The operation of connecting the output to the first input of the differential amplifier serves to adjust the working-point of the differential amplifier. During the second time period, the sum of the second output voltage and the voltage adjusted at the first capacitor during the first time period is applied to the input of the differential amplifier. The differential amplifier produces an output voltage in dependence upon the voltage at the first input. The output voltage of the differential amplifier has a high or a low voltage state.
The switch arrangement of the comparison circuit, which consists of two parallel-connected switches, is constituted by MOS field effect transistors. Such transistors do not constitute an ideal switch. While the switches are closed (the transistors are conducting), a charge is stored in the channel of the respective transistor. When the switch is opened (the transistors are cut off), the charge stored in the channel flows away. In the comparison circuit, an additional charge is supplied to the first capacitor due to the fact that the charge of the first switch flows away. Because of this additional charge, a false voltage comparison can be obtained, especially in the case of voltages to be compared which deviate slightly from each other. In fact, instead of a high and low signal state, respectively, a low and high signal state, respectively, occurs in the output signal of the differential amplifier. As a result, during the analog-to-digital conversion, a digital signal is produced whose value does not accurately correspond to the analog input signal.